The present invention is related to the field of modulators and demodulators of the type that are used in high speed digital data communication systems and, more particularly, to communication systems of the spread spectrum type wherein a pseudo-random (PR) or pseudo-noise (PN) sequence signal modulates a carrier signal in conjunction with a modulating data signal to effect a spread of the spectrum of the carrier signal.
Spread spectrum communications systems utilize a transmission bandwidth many times as large as the information bandwidth in order to achieve jam resistance and to otherwise receive the data signal against decoding. An additional advantage includes multi-path signal rejection. To spread the spectrum of the carrier signal, a PN sequence of binary pulses that are linear larger or non-linear, is used to modulate the phase of a carrier signal either before, or after, modulation by the data signal. Modulation of the carrier signal by the PN signal spreads the bandwidth of the carrier signal to that of the PN signal, the PN signal having a much greater bandwidth than the data signal. A difficulty arises when a mixer circuit of the double-balanced type is used as the modulating means in the transmitter and/or as the demodulating means in the receiver. The task of attempting to match the electrical characteristics of all the like components forming the double-balanced mixer so as to provide a perfectly symmetrical output signal when a symmetrical input signal is received as the modulating signal is extremely difficult. In actuality, what occurs, is that for a symmetrical modulating signal, the output signal is not symmetrical both in amplitude and in phase. This imperfection and/or imbalance causes, for example, in the transmitter a carrier spectral line to be present in the transmitted signal. This spectral line will allow relatively easy detection of the data signal by an intruder. Conversely, in the receiver, the presence of a carrier spectral line having a power level that exceeds adjacent spectral lines, reduces the protection afforded by the circuit against a synchronous, continuous wave (CW) jammer or a CW jammer with a frequency offset less than half of the receiver's IF strip bandwidth. The effect then is that the carrier spectral line present in the received signal reduces the processing gain of correlators and/or demodulators used within the receiver.
As a further discussion of the operation of a spread spectrum type receiver, the received spread spectrum signal is correlated with a replica of the PN sequence that was used to modulate the carrier signal to collapse the signal back to its original data bandwidth. Once the signal is back to the data bandwidth it is detected, generally, by a phase lock loop.
For a general discussion of spread spectrum techniques which are utilized against interferring waveforms and/or detection, reference is made to "A Discussion of Spread Spectrum Composite Codes" by D. J. Braverman, dated Dec. 1, 1963, and available from the Defense Documentation Center as AD No. 425862. Two patents of interest for showing the state of the art as regards spread spectrum communications systems are U.S. Pat. No. 3,478,268 entitled "Suppression of Strong Interferring Signals In A Radio Receiver", by G. J. Coviello; and U.S. Pat. No. 3,916,313 entitled "PSK-FSK Spread Spectrum Modulation/Demodulation", by R. B. Lowry. Both of the aforereferenced patents deal with a PN sequence being used to broaden the spectrum of a carrier wave.
The present invention eliminates the difficulty associated with attempting to compensate for the imbalance in the electrical characteristics of the mixer.